pfhub -> fipy in cachix call

Author: wd15

.github/workflows/nix.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: cachix/install-nix-action@v20
         with:
           nix_path: nixpkgs=channel:nixos-unstable
-          name: pfhub
+          name: fipy
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
           extraPullNames: nix-community
       - run: echo "UID=${UID}" >> $GITHUB_ENV